Kiln Controllers
Introduction
Like everything in this golden age of technology, the means of controlling the temperature inside a kiln has ad-vanced rapidly. We have moved from an “intuitive sense” developed through years of trial and error, to visual inspection of pyrometric cones, to mechanical shut off devices, and now have electronic devices that can do everything but throw the pots for you.
While electronic controllers have been around for quite a while they have only recently come down in price enough to .t into the average potter’s budget. What may have cost thousands of dollars 10 or 15 years ago is now only a few hundred. Because of this drop in price, electronic kiln controllers are no longer reserved for the industrial ceramic engineer. They are used by teachers, home hobbyists, and professional artists all over the world.
What is a Kiln Controller
There are many different types of kiln controllers. Some can control the
atmosphere of the kiln as well as the temperature. We are going to look
at the more basic units that are sold with most top loading electric kilns
and some front loaders.
An easy way to understand how a controller works is to break it down to
its core functions: Sense, Decide and Act.
The thermocouple senses the temperature in the kiln chamber and sends
that information to the controller. The controller takes the information
and compares it to the .ring program input by the operator and makes a
decision to act or not to act. If the program data indicates a need to
act the controller will send a signal for the relays to open or close.
The relays are the components that control the power to the heating elements.
When they are open the power is off and when they are closed the power
is on.
Programming
There are 2 types of programs that can be entered into the controller,
custom designed programs and programs which come with the controller that
are designed to simulate a .ring using cones. These are commonly called
Cone Fire programs.
Custom Designed Programs
Custom designed programs are often referred to as “Ramp and Hold”
or “Ramp and Soak” programs. They all are comprised of one or
more segments. A segment consists of 3 pieces of data, Heating/Cooling
Rate, Ending Temperature, and Hold Time. Below is a 5 segment Cone 10
program that was designed for sculptural pieces with thick walls.
You will notice the .rst segment instructs the kiln to heat up at a rate
of 50°/hr until it reaches 150° F and then holds for 6 hours.
This is an excellent example of how you can customize programs to meet
speci.c needs. Most boards allow 8 segments to use in constructing a program
and 6 spots to save favorite programs in perma-nent memory.
Ramp and Hold programs do not calculate the heatwork for you, therefore
it takes a good understanding of heat-work to design your own program.
Heat work is a measurement that incorporates time and temperature. If
you increase the time of the .ring or the .nal temperature you will increase
the heatwork.
If you look at an Orton Cone Chart you will notice that each Cone value
has a temperature associated with it. Many people do not realize that
this temperature is only valid if the kiln is being .red at a speci.c
rate during the last 150° F of the .ring. Most controllers are calibrated
using the 108°/hr Large Self-Supporting Cone Chart published by The
Edward Orton Jr. Foundation. Therefore if you are .ring slower then 108°/hr,
you will need to adjust the temperature down. Unfortunately determining
how much is still going to take a little trial and er-ror.
Also, it is important to realize that just because you program the kiln
to do something it does not mean that the kiln is capable of achieving
it. If you program a kiln for a rate faster than it can achieve, it will
do only what it can.
Cone Fire Programs
Cone Fire Programs are the most popular because all of the programs are
preloaded into the software of the con-trol board and the user only needs
to load the right one. As mentioned before Cone Fire programs are designed
to simulate the heatwork needed to bend a pyrometric cone. The controller
will automatically adjust the .nal temperature depending on the speed
at which the kiln is .ring
With
the Cone Fire Mode you simply enter the Cone you want to simulate, enter
the speed you wish to .re, and how much time you would like to hold the
temperature at the end of the .ring. It is very common to enter a 5 or
10 minute hold at the end of the .ring to allow the kiln time to balance
out in temperature and give glazes time to .ow.
Prior to controllers it was neces-sary to regulate the heating rate of
the kiln by coming back every so often to advance the switches. With the
Cone Fire programs in the new controllers the kiln will automatically
slow the rate down through critical temperatures. Below is an example
of a Cone 04 , Medium Speed .ring.
As you can see in the graph, the kiln begins at a slow rate to allow water
vapor to escape, slows again through quartz inversion and then again at
the end to insure good cone correlation. This is possible, but dif.cult
and inconvenient to duplicate with a KilnSitter controlled kiln.
For the most part controllers are relatively easy to use and accurate.
Most of the problems associated with con-trollers can be prevented if
the following things are kept in mind:
1) Thermocouples are like heating elements, they wear relative to use
and must be changed on occasion. There are many different types of thermocouples
but the most commonly used is the Type K. When type K thermocouples wear
they begin to error towards an over.re. This is a gradual process and
should be easy to catch by using shelf cones every so often.
2) Relays will need to be replaced on occasion. When one fails it is often
a good time to change all of them.
3) Electronic components are sensitive to excessive heat and moisture.
When controllers were .rst intro-duced to the hobby market they had a
few problems which were mostly associated with heat. Since then, manu-facturers
have been able to design ways of insulating the circuit board from the
heat of the kiln. The problems occur when kilns are placed in tight con.nes
with little circulation. The heat gets trapped in the room and the temperature
rises. If you observe manufacturer’s installation recommendations
you should not have a problem with heat.
4) Do not try to program the kiln beyond it’s capabilities. All kilns
are rated to a set temperature. If the elements are in good shape and
the power is suf.cient, the kiln should achieve the rated temperature
in a reason-able amount of time. When elements begin to wear or there
is a voltage supply problem the kiln will take lon-ger and longer to .re.
The controller will eventually shut off the kiln in order to prevent an
over.re caused by too much heatwork. This normally occurs when the kiln
is not capable of climbing at a rate of at least 12°/hr.
Automatic kilns have opened up a whole new world of freedom to artists.
Freedom to experiment with prod-ucts that are dif.cult to .re such as
crystal glazes and glass and freedom from having to baby sit each .ring.
Kiln controllers have come a long way over the last 15 years they are
more accurate and dependable then ever before. Every year new features
are being developed that can help potters solve .ring problems.
If you don’t own one now, chances are you will in the future.